In systems comprising several mobile units there is often a need for inter-communication between a central system and the mobile units of the system. One example is public transportation traffic routing and information systems, according to which buses deliver brief reports each time they leave a bus stop, for example. Another example is systems for managing commercial traffic according to which lorries and taxis for instance deliver brief reports on their positions and/or reports on accomplished commissions. A typical feature of systems of this kind is that more data is transmitted from the vehicles to the central system than the other way around, and that comparatively brief messages, normally in the magnitude of some hundredths of data bits, are communicated.
According to prior-art technology, a method usually is used according to which a designated path for communication with one of the vehicles is established and according to which information may then be transmitted on that communication path, such as an individually allocated radio channel at the vehicle's disposal during a limited time interval. One of the following two methods usually is employed to determine if and when such a communication path is to be established, viz.                the central system continuously and sequentially polls all vehicles to ascertain whether any one of the vehicles is in possession of information that is to be forwarded to the central system,        via a public channel in the system the vehicles call the central system when they are in possession of information that is to be forwarded to the base station (i.e. an incident-controlled system).        
When the base station has become aware in anyone of these ways that a specific vehicle wishes to communicate, a communication path may be established in some prior-art manner, for instance by allocating to the vehicle a specific radio channel, which is at the disposal of the vehicle for a sufficient length of time to allow the message to be forwarded to the base station and possibly also an acknowledgement message be returned from the base station.
One problem encountered with this technology is the lapse of time before it becomes possible to determine which units are in possession of information to be forwarded. Polling, which is a common method used in public transportation systems, is extremely time-consuming, when a large number of vehicles are included in the system. Quite simply, the inquiries to each discrete vehicle are sparse and infrequent, one consequence of which is that important information may be delayed and if it comes to the worst lack interest by the time it reaches the intended party.
This problem has become particularly acute in conjunction with traffic routing and information systems of the kind that continually gather and provide the passengers with information on the expected arrival times to the various stops. The demands from the systems on reliable and rapid information from the vehicles thus are high, a goal that complex polling routines make it difficult to achieve.
Another problem is that each communication path that is established occupies a part of the (available?) frequency space. Thus, the number of communication paths that can be established simultaneously is limited, which considerably restricts the flexibility and performance of the system. Prior-art systems, such as Mobitex and Tetra, need a comparatively large number of frequencies to achieve the required performance. In most countries, the frequency space is very restricted, and for example public transportation systems often are allocated only a few channels within a narrow frequency band. Obviously, the problem becomes even more serious if the system is to be used for other kinds of traffic as well, such as trucks, taxis, and emergency vehicles and other service cars and the like.
Yet another problem is that a considerable proportion of the available radio capacity of prior-art systems is utilised for administration purposes and for allowing time for switch-overs between transmission and reception in conjunction with the establishment of a communication path with a specific vehicle or with the forwarding and acknowledgement of a message.